Four wheel steering system

ABSTRACT

A closed hydraulic system (22) is provided for transmitting steering movement to a rear wheel steering assembly (16) in response to steering movement of a front wheel steering assembly (14). The front wheel steering assembly is controlled by a steering wheel (18) and a power steering assembly (20). The closed hydraulic system (22) includes a pump assembly (56) and an actuator assembly (58) connected together by conduits (60,62) to define closed fluid flow branches (164,166) for moving a piston (112) in the actuator assembly in response to movement of a piston (78) in the pump assembly. Movement of the actuator piston is transmitted to the rear wheel steering assembly via first and second cam assemblies (122,124) which effect same direction rear wheel steering for rather small front wheel steering angles and which effect opposite direction rear wheel steering for greater front wheel steering angles. Pump assembly (56) includes first and second neutral valves (92,94) for intercommunicating opposite sides of the pump piston (78) with a chamber ( 83) when the piston is in a neutral position. System (22) also includes a reservoir (68) disposed above the normal fluid level of the system and exposed to atmospheric pressure.

FIELD OF THE INVENTION

The present invention relates to four wheel steering. More specifically,the invention relates to apparatus for transmitting steering movement toa rear wheel steering assembly of a vehicle in response to steeringmovement of a front wheel steering assembly.

DESCRIPTION OF THE PRIOR ART

The purpose of four wheel steering is of course to reduce vehicleturning radius and/or to improve high speed maneuverability of thevehicle It is known to employ electrical, electrohydraulic,hydromechanical, or mechanical apparatus to transmit steering movementto rear wheel steering assemblies of vehicles in response to steeringmovement of front wheel steering assemblies. In practice, implementationof such apparatus has been expensive, has been somewhat unreliable,and/or has provided less than optimum performance. Further,hydromechanical and mechanical apparatus has been difficult to packageinto available vehicle space and have been difficult to protect againstthe harsh environment in which vehicles operate.

In a known four wheel steering system, steering movement of a frontwheel steering assembly is transmitted to a rear wheel steering assemblyvia a mechanical apparatus in the form of a rotatably mounted shaftmeans extending longitudinally of the vehicle between the front and rearwheel steering assemblies. The rotatably mounted shaft means sensessteering movement of the front wheel steering assembly and alsotransmits power for steering the rear wheel steering assembly. It isalso known to convert this mechanical apparatus to a hydromechanicalapparatus by connecting the output of the rotatably mounted shaft to avalve which ports pressurized hydraulic fluid from a motor driven pumpto a power cylinder operative to transmit steering movement to the rearwheel steering assembly. It is also known to provide such mechanical andhydromechanical apparatus with a geared transmission operative toreverse the output direction of the rotatably mounted shaft means whenthe steering angle of the front wheel steering assembly exceeds apredetermined amount, thereby changing the steering direction of therear wheels relative to the front wheels from the same direction toopposite direction.

In U.S. Pat. Nos. 4,776,418 and 4,776,419, both of which areincorporated herein by reference, there is disclosed a four wheelsteering assembly including a closed hydraulic system for transmittingsame and opposite direction steering movement to the rear wheels of avehicle in response to steering movement of the front wheels. The closedhydraulic system includes a pump assembly and an actuator assemblyconnected together by conduits to define closed fluid flow branches formoving a piston in a housing of the actuator assembly in response tomovement of a piston in a housing of the pump assembly. The ends of thepistons are sealed by roll diaphragms which limit stroke of the pistons.The piston of the pump assembly is driven by a rack and pin assemblywithin the housing of the pump assembly and the pinion is driven by apivotal linkage arrangement connected at one end directly to a frontwheel steering knuckle arm which can cause erratic movement of the pumppiston due to bounce of the wheel. In the 4,776,418 patent, movement ofthe actuator piston is transmitted directly to the rear wheels via anS-cam assembly which transmits same direction rear wheel steering forrather small front wheel steering angles and then opposite directionrear wheel steering for greater front wheel steering angles. Due to thelimited stroke of the pistons and the arrangement of the S-cam assembly,the system is characterized by relatively high fluid pressure andreaction forces at the cams. The system also includes a rather expensiveand complicated pressurized reservoir to ensure that the roll diaphragmsare always under positive pressure and that fluid fill in the closedfluid flow branches does not vary due to temperature variations and/orleakage.

SUMMARY OF THE INVENTION

An object of the present invention is to improve the closed hydraulicsystem disclosed in the abovementioned patents.

Another object of the present invention is to provide valving forinterconnecting variable volume chambers of a hydraulic cylinder when apiston in the cylinder that varies volumes is in a predeterminedposition.

Another object of the present invention is provided in a closedhydraulic system with the valving and hydraulic cylinders of the aboveobject.

Another object of the present invention is to provide the closedhydraulic system of the above object with a fluid reservoir which isalso interconnected with the variable volumes via the valving.

According to a feature of the invention, a hydraulic cylinder assemblyincludes a housing having a cylinder bore sealed at its ends, a pistonslidably and sealingly disposed in the bore and defining in cooperationwith the housing first and second volumes which varies inversely inresponse to to-and-fro axial movement about a neutral position of thepiston in the bore, a ram affixed to the piston and slidably andsealingly extending through one end of the housing, first and secondports for respectively communicating the first and second volumes with ahydraulic circuit.

The invention is characterized by: first and second neutral valves fixedto with respect to the housing with each valve having a low pressureport normally closed by a valving member and each having a high pressureport; first and second passages for respectively communicating the firstand second valve high pressure ports with the first and second volumes;a chamber for interconnecting the low pressure ports; and actuationmeans operative to move each valving member to the open position inresponse to movement of the piston to the neutral position and to alloweach valving member to move to the normally closed position in responseto a predetermined movement of the piston for the neutral position.

BRIEF DESCRIPTION OF THE DRAWINGS

A four wheel steering apparatus according to the present invention isshown in the accompanying drawings in which:

FIG. 1 is a schematic plan view of a vehicle steering system having theapparatus of the invention disposed between the front and rear wheelassemblies of a vehicle;

FIGS. 2A and 2B are a detailed sectional view of a pump assembly in thesystem of FIG. 1;

FIG. 3 is a sectional view of the pump assembly looking along line 3--3of FIG. 2B;

FIGS. 4A and 4B are a detailed sectional view of an actuator assembly inthe system of FIG. 1;

FIGS. 5 and 6 are sectional views of the actuator assembly respectivelylooking along lines 5--5 and 6--6 of FIGS. 4A and 4B;

FIG. 7 is an enlarged and rolled flat portion of a cylindrical portionof a piston in the actuator of FIGS. 4A and 4B;

FIGS. 8-10, 11A, 11B and 12 illustrate an alternative embodiment of thesystem in FIG. 1 and the components in FIGS. 2A, 2B and 3-7.

DETAILED DESCRIPTION OF THE DRAWINGS

The schematic plan view of FIG. 1 illustrates a steering system 10 for awheeled vehicle having a body and chassis represented by phantom line12, and front and rear steerable wheel assemblies 14,16. The systemincludes a steering wheel 18, a power steering assembly or first means20 for transmitting steering movement of the steering wheel to the frontwheel steering assembly, and a closed hydraulic system or second means22 for transmitting steering movement of the front wheel steeringassembly to the rear wheel steering assembly.

The front wheel steering assembly 14 includes front wheels 24 eachhaving a steering knuckle arm 26 pivotally connected together by tie rodends 28,30 and a steer actuating ram 32 disposed for to-and-fro movementtransverse to the longitudinal direction of the vehicle in known manner.

Steering assembly 20 may be any of several well known mechanicalnon-power or power steering assemblies which transmit steering movementto the front wheel steering assembly in response to steering movement ofthe steering wheel. Assembly 20 is preferably of the power steering typeand, as illustrated therein, is of the well known hydraulic boost typeincluding a source of pressurized hydraulic oil or fluid provided by apump 34 in known manner, a sump 36, and a servo valve 38 for connectingor communicating the source and sump with opposite sides of a piston 32a(FIG. 2A) affixed to ram 32 and disposed in a power steering cylinder 40affixed to the vehicle chassis. The tie rods 28,30 are pivotallyconnected at their opposite ends to one end of ram 32 and the associatedknuckle arm 26. The tie rods move together in to-and-fro motion inresponse to alternate porting or communication of opposite sides ofpiston 32a with the source and sump by valve 38.

The rear wheel steering assembly 16 includes rear wheel assemblies 42supported from the vehicle chassis 12 by pairs of transverse parallelswing links 44,46 and 48,50 which are connected at their outboard orwheel ends to wheel assembly stub axles in known manner, asschematically illustrated for the left rear wheel assembly; thereinwheel ends 44a,46a of links 44,46 are attached to a stub axle assembly52 having an axle portion 52a mounting a wheel 53 for rotation and trackparallel to the longitudinal axis of the vehicle when not being steeredright or left. End 44a is attached in front of axle portion center line52a and end 46a is attached to the rear of the center line. The wheelends of link pair 48,50 are attached to the right wheel assembly 42 inthe same manner. The inboard ends 44b and 48b of links 44 and 48 arepivotally attached to the vehicle chassis 12 in known manner and arerestrained at their pivot axes against movement relative to the chassis.The inboard ends 46b and 50b of links 46 and 50 are attached to a drivenmember 54 transversely movable relative to the chassis but otherwiseretained against movement relative to the chassis in a manner explainedfurther hereinafter.

Closed hydraulic system 22 includes a pump in the form of a hydrauliccylinder assembly 56, an actuator also in the form of a hydrauliccylinder assembly 58, and conduits or passages 60,62 respectivelyconnected at one end to ports 64a,64b defined by a housing assembly 64of the pump assembly and respectively connected at the other end toports 66b,66a defined by a housing assembly 66 of the actuator assembly.System 22 also includes a reservoir 68 having a hydraulic fluid levelexposed to atmospheric pressure and disposed vertically above the restof the closed system 22 and connected thereto by a conduit 70 connectedto a port 64c of the pump housing assembly 64.

Looking now at FIGS. 2A, 2B, and 3, pump assembly 56 includes thehousing assembly 64 affixed to power steering housing 40 by U-bolts 72and having a cylindrical bore 64d with an axis extending parallel to andspaced from the axis of motion of ram 32, ends 74,76 for sealing theends of the bore, a reciprocal piston 78 slidably disposed in the boreand having a ram 78a affixed thereto and slidably extending through anopening 76a in end 76. The interface of opening 76a and the ram issealed by a dynamic seal 77. The piston includes axially oppositelyfacing ends 78b,78c of equal surface area defining variable volumes80,82, axially spaced apart annular lands 78d,78e having annular groovesretaining seals to prevent direct fluid communication between thechambers, and a reduced diameter portion 78f between the annular landswhich forms a chamber 83. A tube 84 of diameter equal to ram 78a issealingly affixed to end 74 and slidably received by a bore 78g in thepiston. The interface of bore 78 g and the tube is sealed by a dynamicseal 86. Ram 78a is threadedly secured at its end 78h to a fork 88received in an annular groove 90a of a member 90 which is threadedlysecured on one side to an end 32b of ram 32 and which defines a ballsocket retaining a ball end 30a of tie rod end 30. This arrangement ofsecuring rams 32 and 78a together and affixing their housing togetherfacilitates unitary assembly and installation of the assembly, readyadjustment of the rams relative to each other, and mitigates relativemovement between the assemblies which can in service effect properadjustment.

Looking now mainly at FIG. 3, the pump assembly also includes twoneutral valve assemblies 92,94. The valves are operative when open tointercommunicate variable volumes 80,82, chamber 83 and reservoir 68when the front wheels are being steered straight ahead and piston 78 isin the neutral position shown. Valve assemblies 92,94 respectivelyinclude ball-type valving members 96,98 disposed in bores 64e,64fextending radially of the axis of bore 64d and defining conical valveseats 64g,64h concentric to low pressure ports 64m,64n opening intochamber 83, springs 100,102 biasing the balls against the seats with aportion of each ball extending into chamber 83, threaded plugs 104,106closing the bores and reaching the springs, and high pressure portsrespectively communicating bores 64e,64f with ports 64a,64b via passages64p,64q which in turn respectively communicates with variable volumes80,82. Port 64c communicates reservoir 68 directly with chamber 83. Aradially outwardly projecting portion 78k of piston 78 contacts andunseats the balls when the piston 78 is within a predetermined distanceof its neutral position, e.g., a distance corresponding to aboutone-half degree of front wheel steer angle. Accordingly, when theneutral valves are open, reservoir 68 provides a head of fluid pressurefor maintaining full fill of the hydraulic components in the closedhydraulic system.

Looking now at FIGS. 4A, 4B and 5-7, actuator assembly 58 includes thehousing assembly 66 affixed to chassis 12 in any of several known butunshown manners, the housing member 66 including a cylindrical centerbore 66c having an axis parallel to and spaced from the axis of drivenmember 54, bores 66d,66e of larger diameter defining shoulders 66f,66hat their terminus adjacent the ends of the center bore, ends 108,110 forsealing the ends of the bore, a reciprocal piston 112 slidably disposedin the center bore and having axially aligned bores 112a,112b thereinseparated by a wall 112c, tubes 114,116 sealingly affixed to ends108,110 and slidably received by the bores 112a,114a in the piston,spring assemblies 118,120 biasing the piston in opposite directions tothe shown neutral position wherein piston shoulders 112d,112e align withshoulders 66f,66h, and cam assemblies 122,124 for effecting to-and-frorotation of the piston and to-and-fro axial movement of driven member 54in response to to-and-fro axial movement of the piston. The springassemblies include needle thrust bearings 126,128 to allow friction freerotation of the piston relative to the springs. The oppositely facingsurfaces of piston wall 112c define the hydraulic working surfaces ofpiston 112 and define variable volumes 130,132 which respectively varyinversely with respect to variation in volumes 82,80 of the pumpassembly. Piston wall 112c may include two relief valves 134,136 torelieve excessively high pressures in the system to prevent loss offront wheel steering. Dynamic seals in grooves 114a,116a of tubes114,116 seal the chambers containing the spring assemblies from fluid invariable volumes 130,132.

Cam assembly 122 includes a helical groove 112f having axially spacedapart and radially extending side walls 112g,112h defining a constanthelix angle formed into the outer cylindrical surface of piston 112, acam follower 138 having an end 138a fixed to housing 66 and an end 138bhaving an anti-friction bearing 140 disposed in the groove. Since groove112f has a constant helix angle, axial movement of the piston in thebore will provide a constant or fixed amount of piston rotation per unitof axial movement of the piston.

Cam assembly 124 includes a continuous helical groove 112k havingaxially spaced apart and radially extending side walls 112m,112ndefining first, second and third portions 142,144,146 of different helixangles (see FIG. 7), and an axially movable cam follower 148 fixed atone end 148a to the driven member 54 at the other end 148b having ananti-friction bearing 150 disposed in groove 112k. Cam follower 148 isretained against circumferential movement relative to the axis of piston112 by a slide block 152 intermediate the follower ends and disposed inan axially extending slide groove defined by aligned openings 66k inhousing 66 and 154a in a housing member 154 affixed to housing 66 by aplurality of bolts 158. Housing member 154 defines a bore 160 allowingsliding movement of driven member 54 along an axis parallel to the axisof piston 112 and transverse to the vehicle chassis. The driven member54 is pivotally secured to transverse links 46,50 via bolts 162.

In FIG. 7, the outer surface 112p of piston is rolled flat to illustratethe angular relation of cam 138,148 are positioned in the respectivegrooves at positions corresponding to the neutral position of therespective cam assemblies and the piston. In disclosed embodiment, thefirst portion 142 of variable helix groove 112k extends in bothdirections away from the neutral position and has a helix angle lessthan the helix angle of constant helix angle groove 112f, the secondportions 144 extend both ends of portion 142 and have like helix angleswhich are greater than the helix angle of the constant helix anglegroove, and the optional third portions 146 extend the adjacent ends ofportions 144 and have like helix angles which are equal to the helixangle of the constant helix angle groove.

The cam arrangement of FIG. 7 causes cam follower 148 to move axially ina direction opposite to the direction of axial movement of piston 112when follower 148 is in first portion 142, causes follower 148 to moveaxially in the same direction of axial movement of the piston with agreater relative per unit movement when the follower is in portions 144,and causes no further relative movement of the piston and follower 148when in the third portions.

To determine movement of axially movable follower 148 relative to axialmovement of piston 112 it is important to remember that the piston outersurface 112p moves axially and rotates relative to both followers,follower 138 does not move, and followers 138,148 are always 180rotational degrees apart. Accordingly, in FIG. 7, vertical line X₀represents the neutral positions of the cam followers, line pairs R₀₁-R₀₂, R₁₁ -R₁₂ and R₂₁ -R₂₂ are 180 degrees apart. Line pair R₀₁ -R₀₂cross line X₀ at the neutral positions of followers 138, 148,respectively. Line X₁ represents a leftward axial movement of piston 112which causes follower 138 to be positioned at its intersection withlines X₁ and R₁₁ and follower 148 to be positioned at the intersectionof lines P₁ and R₁₂. Since the intersection of lines P₁ and R₁₂ is tothe left of the intersection of lines X₁ and R₁₁, follower 148 has movedleftward relative to fixed follower 138 an amount equal to the distancebetween P₁ and X₁. Line X₂ represents a further leftward axial movementof piston 112 which causes follower 138 to be positioned at theintersection with lines X₂ and R₂₂ and follower 148 to be positioned atthe intersection of lines P₂ and R₂₁. Since the intersection of lines P₂and R₂₂ is to the right of the intersection of the lines X₂ and R₂₂,follower 148 has moved rightward relative to fixed follower 138.

As thus far described, conduit 60, pump volume 80 and actuator volume132 define a first closed fluid flow branch 164; and conduit 62, pumpvolume 82 and actuator volume 130 define a second closed fluid flowbranch 166. Since the branches are filled with a non-compressiblehydraulic oil, sliding movement of pump piston 78 in response tosteering movement of the steer assembly ram 32 causes fluid flow in bothbranches and corresponding sliding movement of actuator piston 112 withresultant movement of the rear transverse links 46,50 which causes therear wheels to pivot about their pivotal connection to the fronttransverse links 46,48. Should the steering wheel be turned to effect aleft steer angle of the front wheels, ram 32 and pump piston 78 willmove rightwardly, thereby effecting a flow of fluid in branch 164 fromvolume 82 of the pump to volume 130 of the actuator assembly and areverse flow in branch 166 with respect to volumes 80,132. Forrelatively small front steer angles, this flow is sufficient to move thepiston rightwardly only within first portion 142, thereby moving drivenmember 54 rightwardly to effect same direction steer angles of the rearwheels. For relatively large front steer angles, the flow is sufficientto move the piston into second portions 144, thereby reversing thedirection of the driven member to effect opposite direction steering ofthe rear wheels.

Looking now at FIGS. 8-12, therein is an alternative embodiment 200 ofrear steering system 22 shown in FIGS. 1-7. In the alternativeembodiment, the front steering assembly is as shown in FIG. 1, the rearwheels 202 as shown in FIG. 8 are steered by steering knuckles 204 andtie rod ends 206 analogous to the front steering knuckles and tie rodends. A modified pump assembly 208 shown in FIGS. 9 and 10 embodies thebasic inventive features of pump assembly 56. A modified actuatorassembly 210 shown in FIGS. 11A, 11B and 12 embodies the basic inventivefeatures of actuation assembly 58. The pump and actuator assembly ofboth embodiments are interchangeable.

With brief reference to FIG. 8, actuator assembly 210 is secured to thevehicle chassis in a manner analogous to that of actuator assembly 58,and includes a housing 212 having a through ram 214 pivotally secured atits ends 214a,214b to tie rod ends 206 in a manner analogous to that ofram 32 in FIG. 2B. Housing 212 includes ports 212a,212b connected toconduits 216,218 respectively analogous to conduits 60,62 of system 22.

Pump assembly 208, as shown in FIGS. 9 and 10, includes a housing 220affixed to the power steering housing 40 by U-bolts 76 and having acylindrical bore 220a with an axis extending parallel to and spaced fromthe axis of motion of ram 32, ends 222,224 for sealing the ends of thebore, a reciprocal piston 226 slidably disposed in the bore and havingrams 226a,226b affixed thereto and slidably extending in oppositedirections through openings 222a,224a in the ends 222,224. The pistonincludes axially oppositely facing hydraulic working surfaces 226c,226dof equal surface area defining variable volumes 228,230, communicatingdirectly with ports 220b,220c and annular grooves retaining seals 232 toprevent direct fluid communication between the variable volumes. Housing220 includes a second bore or chamber 220d with an axis extendingparallel to and spaced from the axis of bore 220a, ends 234,236 forsealing the ends of the bore, a rod 238 slidably and sealingly extendingthrough end 236. An end 238a of the rod external of bore 220d isthreadedly secured to the fork 88 which is secured to ram 226b andreceived by groove 90a in the same manner as in FIG. 2B. The other endof rod 238 includes a radially outwardly projecting portion 238banalogous to portion 78k in FIGS. 2B and 3.

Housing 220 also includes two neutral valve assemblies 240,242,functionally and structurally the same as valves 92,94, and havingball-type valving members 244,246 which, when seated, project intochamber 220d and are unseated by projecting portion 238b of rod 238 whenpiston 226 is in the neutral position. Chamber 220d continuouslycommunicates with the reservoir 68 via a port 220e and a conduit 248.Variable volumes 228,230 continuously communicate with high pressureports of valves 240,242 via passages 220f and 220g.

Looking now at actuator assembly 210 in FIGS. 11A, 11B and 12, theactuator includes the housing 212 affixed to the chassis in a manneranalogous to that of actuator assembly 58. The housing includes acylindrical bores 212c,212d, ends 250,252 for sealing the ends of thebores, a reciprocal piston 254 slidably disposed in bore 212c and havinga through bore 254a with the ram 214 slidably extending therethrough, apin 255 fixed at both ends to housing 212 and extending through a slot214c in the ram to prevent rotation of the ram, a double acting springassembly 256 biasing the piston in opposite directions to the shownneutral position wherein piston shoulder 254b aligns with a shoulder212e, and cam assemblies 258,260 for effecting rotation of the pistonand to-and-fro axial movement of ram 214 in response to to-and-fro axialmovement of the piston. The piston includes axially oppositely facingends of equal surface area defining volumes 262,264 which vary inverselyin volume with respect to variations in volumes 228,230 of the pumpassembly. The spring assembly 256 includes a cylindrical member 266having slotted openings 266a and radially extending flanges 266b,266c atits opposite ends, a spring retainer 268 for limiting rightward axialmovement of member 266, a retainer/anti-friction thrust bearing 270 forpulling member 266 rightward with piston 254 and for allowing rotationof the piston relative to the member, and an anti-friction thrustbearing 272 for allowing rotation of the piston relative to a spring 274sandwiched between the bearing and flange 266b. The left interface ofpiston 254 with housing bore 212c is sealed by dynamic seals 276, andthe right interface of piston 254 with ram 214 is sealed by dynamicseals 280. This seal arrangement allows system fluid to lubricate thesliding interfaces and the cam assemblies.

Cam assemblies 258,260 may be the same as cam assemblies 122,124 whereinthe fixed cam follower reacts in the cam groove of constant helix angleand the axially movable cam follower reacts in the cam groove ofvariable helix angle. However, herein a fixed cam follower 282preferably reacts in a cam groove 284 of variable helix angle and anaxially movable follower 286 reacts in a cam groove 290 constant helixangle. This reversed roll of the cam grooves requires the first portionof groove 284 to have a helix angle greater than the helix angle of thegroove 290 and the second portion groove 284 to have a helix angle lessthan the helix angle of groove 290.

Actuator assembly 58 may also be provided with two relief valves 292,294to relieve excessively high pressures in the system the valves are shownenlarged in FIG. 12. Valve 292 includes a spring loaded ball-typevalving member 296 for relieving pressure in volume 262 and valve 294includes a spring loaded ball-type valving 298 member for relievingpressure in chamber 264. The respective inlet and outlets 300,302 of thevalves freely communicate with volume 264 via radial spacing betweenparts of housing bore 212c and piston 254.

A preferred embodiment of the invention has been disclosed herein forillustrative purposes. Many variations and modifications of thedisclosed embodiment are believed to be within the spirit of theinvention. The following claims are intended to cover inventive portionsof the disclosed embodiment and variations and modifications believed tobe within the spirit of the inventive portions.

What is claimed is:
 1. A hydraulic cylinder assembly including a housinghaving a cylindrical bore at its ends, a piston slidably and sealinglydisposed in the bore and defining in cooperation with the housing firstand second volumes which vary inversely in to to-and-fro axial movementabout a neutral the piston in the bore, a ram affixed to the an slidablyand sealingly extending through one e of the housing, first and secondports for respectively communicating the first and second volumes with ahydraulic circuit; characterized by:first and second neutral valvesfixed with respect to the housing, each valve having a low pressure portnormally closed by a valving member and a high pressure port; first andsecond means for respectively communicating the first and second valvehigh pressure ports with the first and second volumes; chamber means forintercommunicating the low pressure ports; and actuation means operativeto move each valving member to the open position in response to movementof the piston to the neutral position and operative to allow eachvalving member to move to the normally closed position in response to apredetermined movement of the piston from the neutral position.
 2. Thecylinder assembly of claim 1, wherein each neutral valve includes avalve seat circumscribing the low pressure port, a spring biasing thevalving member against the valve seat, and a portion of the valvingmember protruding through the low pressure port and into the chambermeans; and wherein the actuation means is operative to contact theprotruding portion of each valving member and unseat the valving memberwhen the cylinder assembly is in the neutral position.
 3. The cylinderassembly of claim 2, wherein each valving member is of the ball type. 4.The cylinder assembly of claim 2, wherein the piston includes axiallyspaced apart seal assemblies cooperating with the cylindrical wall ofthe bore and a reduced diameter portion between seal assemblies definingthe chamber means, the protruding portion of each valve member extendingradially inward into the reduced diameter portions, and the actuationmeans being defined by radially outward extending portions of thepiston.
 5. The cylinder assembly of claim 4, wherein each valving memberis of the ball type.
 6. The cylinder assembly of claim 2, wherein thehousing includes a second bore sealed at its ends, being radially spacedfrom and parallel to the cylindrical bore, and defining the chambermeans; a second ram having an end portion disposed in the second boreand defining the actuation means with the other end slidably and sealingextending through one end of the housing and affixed for movement withthe ram affixed to the piston.
 7. The cylinder of assembly of claim 6,wherein each valving member is of the ball type.
 8. A steering appartusfor a wheeled vehicle having front and rear dirigible wheel assemblies,the apparatus comprising:first means for transmitting left and rightsteering movement to a front wheel steering assembly in response to asteering wheel being turned left and right from a neutral position toeffect left and right steering angles of the front wheels, second meansfor transmitting steering movement to a rear wheel steering assembly inresponse to steering movement of the front wheel steering assembly; thesecond means characterized by: a closed hydraulic system including firstand second closed fluid flow branches comprising a fluid displacementassembly operative to effect fluid flow in both branches in response tosteering movement of the front wheel steering assembly; a hydraulicactuator assembly operative to effect steering movement of the rearwheel steering assembly in response to said fluid flow, the fluid in thebranches alternately increasing from a low pressure to an operationalpressure in response to alternate steering movement of the front wheelsteering assembly; the fluid displacement assembly including: ahydraulic cylinder assembly including a housing having a cylindricalbore sealed at its ends, a piston slidably and sealingly disposed in thebore and defining in cooperation with the housing first and secondvolumes which vary inversely in response to to-and-fro axial movementabout a neutral position of the piston in the bore, a ram affixed to oneend of the piston and slidably and sealingly extending through one endof the housing, first and second ports for respectively communicatingthe first and second volumes with the first and second fluid flowbranches; characterized by: first and second neutral valves fixed withrespect to the housing, each valve having a low pressure port normallyclosed by a valving member and a high pressure port; first and secondpassage means for respectively communicating the first and second valvehigh pressure ports with the first and second volumes; chamber means forintercommunicating the low pressure ports; and actuation means operativeto move each valving member to the open position in response to movementof the piston to the neutral position and operative to allow eachvalving member to move to the normally closed position in response to apredetermined movement of the piston from the neutral position.
 9. Thesteering apparatus of claim 8, wherein the closed hydraulic systemincludes a hydraulic fluid reservoir having a normal fluid level exposedto atmospheric pressure, disposed vertically above the fluid flowbranches and in continuous communication with the chamber means.
 10. Thesteering apparatus of claim 8, wherein each neutral valve includes avalve seat circumscribing the low pressure port, a spring biasing thevalving member against the valve seat, and a portion of the valvingmember protruding through the low pressure port and into the chambermeans; and wherein the actuation means is operative to contact theprotruding portion of each valving member and unseat the valving memberwhen the cylinder assembly is in the neutral position.
 11. The steeringapparatus of claim 10, wherein each valving member is of the ball type.12. The steering apparatus of claim 10, wherein the closed hydraulicsystem includes a hydraulic fluid reservoir having a normal fluid levelexposed to atmospheric pressure, disposed vertically above the fluidflow branches and in continuous communication with the chamber means.13. The steering apparatus of claim 10, wherein the piston includesaxially spaced apart seal assemblies cooperating with the cylindricalwall of the bore and a reduced diameter portion between the sealassemblies defining the chamber means, the protruding portion of eachvalve member extending radially inward into the reduced diameterportions, and the actuation means being defined by radially outwardextending portions of the piston.
 14. The steering apparatus of claim13, wherein each valving member is of the ball type.
 15. The steeringapparatus of claim 13, wherein the closed hydraulic system includes ahydraulic fluid reservoir having a normal fluid level exposed toatmospheric pressure, disposed vertically above the fluid flow branchesand in continuous communication with the chamber means.
 16. The steeringapparatus of claim 10, wherein the housing assembly includes a secondbore sealed at its ends, being radially spaced from and parallel to thecylindrical bore, and defining the chamber means; a second ram having anend portion disposed in the second bore and defining the actuation meanswith the other end slidably and sealing extending through one end of thehousing and affixed for movement with the ram affixed to the piston. 17.The steering apparatus of claim 16, wherein each valving member is ofthe ball type.
 18. The steering apparatus of claim 16, wherein theclosed hydraulic system includes a hydraulic fluid reservoir having anormal fluid level exposed to atmospheric pressure, disposed verticallyabove the fluid flow branches and in continuous communication with thechamber means.
 19. A closed hydraulic system comprising:first and secondhydraulic cylinder assemblies each including a housing having acylindrical bore sealed at its ends, a piston slidably and sealinglydisposed in the bore and defining in cooperation with the housing firstand second volumes which vary inversely in response to to-and-fro axialmovement about a neutral position of the piston in the bore, means forintercommunicating the first volumes and means for intercommunicatingthe second volumes, the first hydraulic cylinder assembly including aram affixed to the piston and slidably and sealingly extending throughone end of the housing; characterized by: the first hydraulic cylinderassembly including neutral valve means fixed with respect to the housingof the first hydraulic cylinder assembly, the valve means having lowpressure port means normally closed by a valving means and a highpressure port means, passage means for communicating the high pressureport means with the first and second volumes of the first hydrauliccylinder assembly, chamber means communicating with the low pressureport means, and actuation means operative to move each valving means tothe open position in response to movement of the piston to the neutralposition and operative to allow the valving means to move to thenormally closed position in response to a predetermined movement of thepiston from the neutral position; and the second hydraulic cylinderassembly including first cam means reacting between the housing andpiston of the second hydraulic cylinder assembly for effectingto-and-fro rotation of the piston in the bore thereof about a neutralposition of the cam means in response to to-and-fro axial movement ofthe piston about the neutral position of the piston driven means, andsecond cam means reacting between the piston and the driven means of thesecond hydraulic cylinder assembly for effecting to-and-fro movement ofthe driven means in response to the rotation of the piston.
 20. Theclosed hydraulic system of claim 19, wherein the neutral valve meansincludes first and second valves each having a low pressure portdefining the pressure port means, a valving member defining the valvingmeans, and a high pressure port defining the high pressure port means,first and second passages defining the passage means respectivelycommunicating the high pressure ports of the first and second valveswith the first and second volumes, the chamber intercommunicating thefirst and second low pressure ports, and the actuation means operativeto move each valving member to the open position in response to themovement of the piston to the neutral position and operative to alloweach valving member to move to the normally closed position in responseto the predetermined movement of the piston from the neutral position.